1. Field of the Invention
This invention relates to a method for manufacturing variable capacitance pressure transducers and to the structure of the pressure transducer.
2. Prior Art
U.S. Pat. No. 4,415,948 teaches bonding of two highly doped silicon wafers, one with an etched cavity, together by using an intermediary glass coating deposited on one wafer. The other wafer is sealed to the coated wafer by electrostatic bonding.
U.S. Pat. No. 4,420,790 teaches forming a cavity by putting silicon spacers between two silicon plates that are covered with passivation layers of silicon dioxide and by a combination of silicon dioxide and silicon nitride layers. These layers electrically isolate the capacitor electrodes. The sensor cavity is not hermetically sealed but is open to the ambient. The two silicon plates are joined together by solder bumps and the spacing between the plates is kept constant by the spacers.
U.S. Pat. No. 4,424,713 teaches making a sensor structure including plates of glass, silicon and glass. The silicon is sealed to the glass by electrostatic bonding. The silicon plate has cavities on both sides, the cavity on one side being bigger than on the other side in order to provide stress relief on the silicon diaphragm.
U.S. Pat. No. 4,390,925 teaches making a sensor with multiple interconnected cavities in a silicon plate bonded electrostatically to a glass plate. Such a structure is taught to be a high pressure sensor.
U.S. Pat. No. 4,184,189 teaches making a sensor with two metallized glass plates bonded together by a sealing glass mixture at about 500.degree. C. The spacing between plates is achieved by shim stock.
U.S. Pat. No. 4,207,604 teaches making a sensor with a pair of insulating metallized plates sealed by melting glass frit. A metallized guard ring for the capacitor is used.
U.S. Pat. No. 4,345,299 teaches using two metallized ceramic substrates with an annular sealing ring to form a capacitive pressure transducer.
U.S. Pat. No. 4,177,496 teaches metallizing a pair of thin insulating plates (alumina quartz, pyrex) to form a capacitor. Glass frit or ceramic base material is fired to form a seal.
U.S. Pat. No. 4,261,086 teaches making a sensor by using a silicon wafer with etched cavities and anodically bonding it to a glass substrate. The capacitor electrodes are formed by metallizing the glass and highly doping the silicon cavity surface. Electrical connections are made through metallized holes in the glass.
U.S Pat. No. 4,386,453 teaches making a sensor by using a silicon wafer with etched cavities anodically bonded to a glass substrate. There is metallization of holes drilled in the glass. The metallized holes are used as electrical feedthroughs to make contact to capacitor plates.
Even though there are known capacitive-type pressure sensors, there continues to be a need to have a pressure sensor which can be produced with a fewer number of steps, which can be a smaller size, which has reduced unit cost, has improved accuracy and reliability, does not require temperature compensation of the signal and is not adversely effected by thermocycling. These are some of the problems this invention overcomes.